If a particle at rest (with mass 'm' and weight 'mg') begins to fall under the influence of gravity, its velocity is increased initially over a period of time. The particle is subjected to the resistance of the surrounding water through which it descends. This resistance increases with particle velocity until the accelerating and resisting forces are equal. From this point, the solid particle continues to fall at a constant maximum velocity, referred to as the terminal velocity, ut. You should recall from the last chapter that we called the settling velocity. The force responsible for moving a spherical particle of diameter'd' can be expressed by the difference between its weight and the buoyant force acting on the particle.
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